Before entering the combustion chamber, petrol is mixed with air in the caeburettor, or fuel-injection system. The normal quantity of air, called the 'stoichiometric' quantity, is exactly enough to combine with the fuel.

If the fuel is in excess, the resulting 'rich' mixture produces large amount of carbon monoxider and unburnt hydrocarbons. This is the thing that you will get when the car engine is 'choked' for a cold start in cold winter mornings.

Car manufactures produces engines that have excess air mixed with the fuel. This 'lean' mixture gives reduced emissions of carbon monoxide and hydrocarbons.

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